Sound source, medium and the audible range
Sound is produced by a vibrating source -- a loudspeaker cone, a guitar string, a drum skin, vocal cords. The vibration is passed on from particle to particle, so sound needs a medium (solid, liquid or gas) and cannot travel through a vacuum. A healthy human ear hears 20 Hz to 20 kHz (20 000 Hz). Ultrasound is sound of frequency greater than 20 kHz, above that upper limit; bats and dolphins produce and hear it.
Speed of sound from distance / time
Speed = distance / time, measured in m/s. In air the speed of sound is about 330 to 340 m/s -- roughly a million times slower than light, which is why a lightning flash is seen before the thunder is heard. An echo is sound reflected from a hard flat surface; the sound goes out to the reflector and back, so the distance is twice the distance to it: speed = 2d/t, or d = (speed x t)/2 when finding the gap.
Amplitude sets loudness, frequency sets pitch
Loudness depends on amplitude (bigger amplitude = louder); pitch depends on frequency (higher frequency = higher pitch). The two are independent -- a note can be loud and low, or quiet and high. (Extended) A sound wave is longitudinal: particles vibrate along the direction of travel, giving compressions (high pressure) and rarefactions (low pressure). (Extended) Sound travels fastest in solids, slower in liquids, slowest in gases.
Drawn from real examiner reports.
Audible range is 20 Hz to 20 kHz
The mark scheme wants both ends, with units: 20 Hz to 20 kHz (20 000 Hz). Versions such as "2 Hz to 20 kHz" or "20 Hz to 2 kHz" score zero -- one end is a factor of ten out. Learn the pair together, and remember that 20 kHz is the upper limit, so anything above it is ultrasound rather than the faintest audible sound.
Flagged Jun 2022 P31 Q6d; Jun 2023 P31 Q12ciii
Ultrasound needs the 20 kHz boundary
Ultrasound must be defined as sound of frequency greater than 20 kHz -- above the upper limit of human hearing. "Very high-pitched sound", "sound humans cannot hear" and "the sound used in hospital scans" are all too vague to earn the definition mark. Quote the number and the unit every time you are asked to define it.
No medium means no sound
Sound is carried by vibrating particles, so it needs a solid, liquid or gas and cannot cross a vacuum. The mark is for the absence of a medium/particles -- not "the sound is outside the audible range", "it is too quiet" or "it is too far away". Light needs no medium, which is why space can be seen but never heard.
Flagged Jun 2023 P32 Q12; Nov 2022 P32 Q9ciii
Loudness is amplitude, pitch is frequency
Keep the two pairs apart: a larger amplitude = LOUDER (more energy carried), a higher frequency = HIGHER pitch. Candidates cross-wire them, or claim a bigger amplitude makes the sound quieter. Memory hook: amplitude -> amount of energy -> loudness. Questions change one and hold the other fixed -- "louder at the same pitch" means bigger amplitude, same frequency.
Flagged Nov 2023 P13 Q34
Echo distance is 2d, not d
An echo is sound reflected back, so it covers the gap to the reflector twice. Use speed = 2d/t, and going the other way d = (speed x t)/2. Dividing by t without doubling -- or quoting the round-trip distance as the distance to the wall -- puts the answer out by a factor of two and loses the answer mark.
Flagged Nov 2023 P31 Q3bii
(Extended) Solids fastest, gases slowest
The order is solid > liquid > gas -- sound is fastest in a solid and slowest in a gas. Candidates very often reverse it and claim sound is fastest in air. The reason: solid particles are closest together and most strongly bonded, so each passes the vibration on almost immediately, while gas particles are far apart and pass it on slowly.
Flagged Jun 2023 P43 Q3cii; Nov 2022 P42 Q6bi
(Extended) Sound is longitudinal, not transverse
A sound wave is longitudinal: particles vibrate back and forth along the direction of travel, giving compressions and rarefactions. A water wave is transverse -- vibration at right angles to travel. Many candidates swap these round, calling sound transverse or a water wave longitudinal. Name the wave type AND the direction of vibration.
Flagged Jun 2023 P43 Q3cii; Nov 2022 P42 Q6bi
Write speed = distance / time first
Every speed-of-sound question on this leaf uses the distance/time method -- the wave equation belongs to P3.1, not here. Write the relationship, substitute, then answer. A formula built out of units, or an upside-down rearrangement, loses the method mark.
Ask: did the sound come back?
Before substituting, decide whether the path is one-way or there-and-back. An echo from a wall or cliff means the distance is 2d; a sound crossing a field to a second student is a single d. The wording of the stem -- not the numbers -- tells you which.
Metres, seconds, and the unit m/s
Convert every distance to metres and every time to seconds before substituting, then always write the unit on the answer: m/s for a speed, m for a distance, s for a time. A bare number loses the unit mark, and converting after the arithmetic is the classic slip.
Match the command word
State the audible range -> 20 Hz to 20 kHz, both ends with units. Define ultrasound -> greater than 20 kHz. Explain the vacuum -> no particles to carry the vibrations. Describe the longitudinal nature -> compressions AND rarefactions, both halves needed.
Cambridge 0654 spec reference: Section P3 "Waves", sub-topic P3.4 (Core + Extended). This leaf covers the production of sound by vibrating sources, the audible range (20 Hz to 20 kHz), the need for a medium, determining the speed of sound by a distance/time method, the effect of amplitude on loudness and frequency on pitch, echoes as reflected sound, and the definition of ultrasound (Core); plus the longitudinal nature of sound as compressions and rarefactions, and the ordering of the speed of sound in solids, liquids and gases (Extended, labelled (E)).
(Core) Speed of sound -- the distance/time method: where = speed of sound (m/s), = distance travelled (m), = time taken (s).
(Core) Echo (reflected sound): the sound travels to the reflector and back, so the distance is doubled: where = distance to the reflector.
There is no wave equation in this leaf -- that belongs to P3.1 (general wave properties). Here the speed of sound is found only by distance time.
(Core) Define ultrasound.
A student stands 660 m from a large flat cliff. She fires a starting pistol and hears the echo 4.0 s later. Calculate the speed of sound in air. (3 marks)
